The present disclosure relates generally to compensation for facsimile transmission through several types of communication networks, and relates more particularly to adaptive facsimile timers that can compensate for timing issues encountered in facsimile transmission through several types of communication networks.
Facsimile document transmission continues to have an important role in business communications for a number of reasons, including the ability to transfer images not stored on a local computer, legal acceptance of handwritten signatures, real-time confirmation of receipt, confidence in what has been sent/received, and the ability to provide a ‘tamper resistant’ copy of the information transferred. Since facsimile-enabled devices can be used with existing telecommunications networks, their popularity has increased to the point where such devices enjoy the advantage of being ubiquitous and familiar to users on a global scale. Such devices also may be shared by a number of individuals, such as in a workplace environment, reducing costs while still allowing for the sending and receiving of documents to be relatively efficient among a group of persons.
While facsimile communications have previously been implemented over circuit switched networks, such as the publicly switched telephone network (PSTN), packet switched networks, such as Internet Protocol (IP) networks, have been implemented to carry communications including facsimile communications. As these different types of networks continue to coexist, translation and communication between them has become (and should continue to be) an important part of communications, including facsimile communications.
IP networks are inherently asynchronous; they also have a higher delay and are relatively ‘lossy’ (lose or drop packets) as compared to PSTN networks, which typically operate on a time-division multiplexed (TDM) basis. While these characteristics of IP networks are known to adversely impact both voice and facsimile communications, the impact to facsimile communications is typically more pronounced. Various solutions have been provided to address drawbacks related to IP network communications; however, such solutions have tended to be focussed on voice data and in many cases can cause more problems than they solve. Facsimile users thus have sometimes reported negative experiences when attempting to perform voiceband (non-T.38) facsimile transmissions over packet switched networks.
Translation between circuit-switched and packet-switched communication networks typically involves the use of translation between different protocols, and is often performed by gateways, sometimes referred to as IP media gateways. A gateway can carry different types of communications between various network types, such as an IP network and the PSTN. Such different types of communications may include voice or facsimile, for example. The gateway typically provides protocol translation service between the networks for these different types of communications. Facsimile transmissions typically adhere to the International Telecommunication Union (ITU) T.30 specification, and are often implemented using the realtime facsimile transmission specification under the ITU T.38 specification. The ITU T.30 and T.38 specifications are hereby incorporated herein by reference.
In a packet switched network that carries voice over IP (VoIP) communications, for example, individual blocks of data are transported with varying propagation delay depending upon the route taken and network conditions at the time, sometimes referred to collectively as “jitter.” Compensation for jitter can be provided at a receiving end or midpoint of a network transmission path by providing sufficient overall throughput delay to accommodate the range of propagation delays, often implemented with a jitter buffer in a network component such as an IP media gateway. Individual packets that have been delayed sufficiently to fall outside of a range that can be accommodated by a given jitter buffer are considered lost or dropped.
The size of the jitter buffer is an important design consideration in constructing network components or networks in general. For example, a network component that implements a relatively large jitter buffer, with an attendant large overall delay, provides a greater tolerance to jitter and packet delays. However, if the jitter buffer size provides a significant overall delay, the result can be uncomfortably long pauses, which in turn can cause both parties to attempt to speak at the same time. Overall delays may also be observed on satellite connections, as a result of retry strategies between endpoints with a connection path that may span several types of communication networks, or when data compression is used, which is typically intended to decrease utilized bandwidth at the expense of added latency.
In general, facsimile transmissions can tolerate a relatively high overall delay in comparison to voice transmissions. However, when there is significant delay present, particularly when accumulated over multiple devices or network components, facsimile transmissions can fail due to the round trip delay exceeding T.30 timeout values. The T.30 facsimile protocol was designed for transmission using the general switched telephone network (GSTN), which in general is a relatively low latency network with a relatively small amount of loss of communication information during transmission. When significant delay is introduced into the communication network path, typical T.30 facsimile traffic can fail, due to the expiration of facsimile timers that are set in accordance with the T.30 specification. In contemporary communication networks, there are a number of sources of delays that can be introduced into communication pathways that cause T.30 facsimile traffic to fail. Some of these delay sources include the introduction of the real time IP facsimile protocol T.38, and network connections that relay or transcode data, which can be present in satellite connections and VoIP networks.
The real time IP facsimile protocol T.38 is relatively resilient to jitter and packet loss, especially if an optional redundancy mode is employed. However, significant delays can result when T.38 is used to implement facsimile over IP (FOIP) service, sometimes on the order of 1-1.5 seconds through a single gateway. The significant delay results from operations conducted in the gateway, including transcoding facsimile data and commands from T.38 IP packets to a PSTN modem generated audio signal, for example. When a facsimile transmission is provided over a network that includes multiple T.38 gateway hops, such as might be experienced with a tier II or III SIP trunk provider, the overall path delay can be on the order of several seconds. The facsimile communication path delay can be amplified due to different manufacturer gateway treatments as result of different manufacturers providing components to implement the communication network path, since delays can vary among the different manufacturer components. Moreover, a T.30 command often is made with the expectation of a response from the command destination, so that there is a roundtrip delay that can easily exceed typical T.30 specified timer constraints.
Some attempts have been made to permit adjustment of T.30 timers for accommodation of facsimile transmission failures due to excessive latency. However, in these conventional techniques, the adjustment of the T.30 timers is manual, so that facsimile transmission settings for facsimile endpoints is individualized on a case by case basis, and the settings are fixed until individually modified, usually by a trained technician on-site. Thus, facsimile transmissions to some endpoints can work with large timer values, whereas transmission to other endpoints do not work, since some endpoints may not have a similarly adjusted timer value. In addition, such manual adjustments may pose a problem for compliance with the T.30 standard, since some timer values may be extended outside the specified range for the T.30 standard.
It would be desirable to overcome the drawbacks related to facsimile transmissions in a communication network with relatively high latency. It would also be desirable to avoid facsimile transmission failure over relatively high latency communication networks.